I really hate to post this but I need some direction. I've only run the OEM Scorpions since I bought the bike in March this year. There are a million threads on the subject but I cant seem to find a definitive (objective) thread on tire comparison. I ride 60/40 street/dirt and ride the bike to work 2 or 3 times a week. My commute is about 80 miles round trip and I can take twisties or highway.
I was really leaning towards the TKC80's but now I'm thinking MEFO Super Explorer's for the mileage. As I understand it, either will be an improvement over the Scorpions off road. I don't ride the wild and wooly with the 990 (I have a 300XCW(e) for that) but I do want to ride more than the NFS roads and Jeep trails that I have been. Suspension has been done, bash plate, guards, side stand relocation all have been installed.
ANYWAY, something I can reasonably expect to get 3-4 thousand miles out of, that aren't just rim protectors is what I'm looking for.
Any help would be appreciated.

I got really good mileage on my rear super explorer, about 8.8k and it had maybe 500-1000 left before the cords but it was handling like shit at that point on the road so I replaced it.

You will loose some cornering ability on the street. I've had my rear end slip on hairpins with the mefo, so keep that in mind. I'd say it's better than the heidenau off-road, but the heidenau is better on-road.

If money was less of an issue I'd just run TKC-80's which give great grip on and off road but they only last 2000-3000 miles on the rear.

I've had two sets each of both TKC80 and Mefo on my old 950A. In my gentle hands, the TKC rear lasted about 3k miles and the front just under 7k miles. The Mefo's lasted 7200 miles at both ends and were totally finished.

I think the Mefo's rode similar to the Scorpions. Quiet with similar grip wet or dry on tarmac. On dirt roads that were dry and full of babyheads, the Mefo front was very good. In mud it was terrible. The Mefo rear was very good all around and lasted so long. Meanwhile the TKC front tire was by far a better dirt tire wet or dry. I found TKC 80's to be poor on wet and cold tarmac. Plus the TKC front was unstable above 90mph.

If I was on a trip and expected to be riding dirt roads, my choice would be a Mefo Super Explorer rear tire with a TKC80 or Pirelli MT21 front tire. If I had no dirt planned I'd opt for the Mefo or Pirelli Scorpions at both ends.

Mefo on the rear. My 950 sits in Mongolia with 11,500 miles on rear, 1/8" from wear bars. Keep the pressure at/near 40PSI if you do alot of high speed tarmac riding. (They come with a small sheet recommending that but it seems to "fall off" them, often.)

IMHO the Heidenau K 60 Scouts are the best allround tire I've used.
Good enough grip in the dirt, nice on the road two up at 100 mph.
Without a doubt the best mileage I've gotten by far.
I've run a set of Mefo's to Alaska, two up fully loaded 990 and
while their mileage was ok, I'm getting 2,000 more miles out of the rears.
Around 7,000 miles.
For comparison I could never get more than 4,000 out of the TKC 80

Went through a couple of sets of K60's this year and have a new set
mounted on the 990 for a ride in South America ( shipping out on
Monday ) got a spare set in the shipping container.

On the pavement I'm pretty aggressive trail braking in the twisties
and the K 60's work as well as can be expected from a 21" front.
No weird squirming around while tipping it in on the brakes.

I've tried most of the combos out there and (for the 60/40 split you mentioned) I've settled on the Mefo SE rear and a Dunlop 606 front as the best combo for me. Not as Grippy as the Scorpions on the street but I can still grind toes in a corner and there's no replacement for a good knobby up front if you're serious about dirt. The TKC front works well but the weak sidewall caused a few rim dings with the stock wheels. The TKC and Rally Raid rears are excellent but wear out way too fast. The Mefo wears like iron but you do lose a little grip in the dirt; a trade off I can live with.

Can't say to much about the TKC tires as I have never used them. I have run a couple of sets of the Mefo tires and really like them alot; and was my choice to come to Central America with. I did try the Kenda Big Block tires and the work very well in the dirt, however on pavement they have lasted about 3k miles. Reports say they are going to roll out new ones with an improved compound so they last a bit longer. I swapped them out just before leaving, so I will have a little tread left to go home to. I would like to try the Heidenu tires sometime to compare them to the Mefo SE. The front Mefo I would like a little more aggressive for the dirt, but overall longevity is great as I got about 7500 miles out of each set, which for me is a better bang for your buck.

I've tried "dirtier" tires on the front (T63 for example) and keep returning to the TKC front - it just works well enough onroad and offroad that I use it and don't sweat it. I do like the 606 on the front when it is new.. but I'm not a fan of them once they start to wear; the 1-2-1 pattern tends to cup the second rows.

908 rear:

MEFO rear:

I'm a "steer with the back end" type rider and I find both of these spin up and go sideways predictably for cornering offroad. None of them last long enough because of this but this is the reason I don't run a TKC in the rear... I can make it go away in a weekend and they aren't cheap.

I don't have suggestions for primarily pavement as imho pavement is just a necessary evil to get to the good stuff... but my understanding is that those who ride more pavement than dirt tend to like the scorps?

the D606 fronts are great if you are going for a weekend ride and want ultimate traction; as mentioned above when they begin to wear things get scary going in to turns on pavement & in the wet. They can be had for about $60 with some looking

The TKC80 front is pretty good offroad, great in gravel so-so on pavement. I ended up with it on my lighter KLR650 where it doesn't have as much work to do and wears well

The Heidy K60 works really well on the street (best dualsport tire I've found as I tend to ride the front hard) and doesn't fill up with mud as fast as the Mefo or Scorpion fronts, which just suck in wet/loamy offroad conditions. I've had to turn around in some places when the Scorpions lose the front every 100 feet. Scotts steering damper or not, they are as slick as a bowling ball.

The Mefo rear wears like iron & when it steps out under hard throttle holds it line instead of sliding out. I roasted the last rear doing some insane burnouts, when it was all done it had hardly affected the carcass & there was still tread left. Other tires that have gotten the same treatment have shown cords within a minute of LC8 abuse. Tough tire, reminds me of old-school Metzelers (hmm, ya think?)

Sounds like a Mefo SE for the back and either a MT 21 or 606 on the front. I've run both the 606 and the MT21 on another bike (KLR) and found the MT 21 to be a bit more predictable with decent mileage.

I've run Mefos for a while on KLR's and just put a Mefo SE on the back of the 990 (TKC80 up front). I love the Mefos but I will try some different fronts. 606 most likely since I want something more "dirty"

I have been amazed at how aggressive the Mefo hooked up in back while still keeping the bike commutable on street. Just ran the bike form Portland OR to LA and it felt great.

Not a huge fan of the Mefo front. Feels weird on street and not to predictable off road. On recommendation from the guys in Alaska, I ran the Mefo front backwards and that helped tremendously especially in mud.

I will keep Mefos on the back and experiment with the front till I find my sweet spot.....